Signal collecting system for radio receivers and the like



Dec. 23, 1941. w. A. SCHAPER ,0 7

SIGNAL COLLEETING SYSTEM FOR RADIO RECEIVERS AND THE LIKE Filed Feb. 19, 1940 2 Sheets-Sheet l 7 INVENTOR mum/2,4 50/4/ 512 ATTORNEY w. A. SCHAPER 2,267,047

Dec. 23, 1941.

SIGNAL COLLECTING SYSTEM FOR RADIO RECEIVERS AND THE LIKE 2 Shets-Sheet 2 Filed Feb. 19, 1940 2 Ewoooob 5 W 9% 0 INVENTOR W/L L IAM A. 50/41 52.

ATTORNEY Patented Dec. 23, 1941 Q William A. Schaper, Cicero, 111., assignor to Johnson Laboratories, Inc., Chicago, Ill., a cor;

poration of Illinois Application February 19, 1944), Serial No. 319,674

8 Claims.

This invention relates to high-frequency circuits; such as those employed in radio receiving systems- More particularly, the invention relates to the portion of such systems which constitutes means for collecting the high-frequency signals radiated from relatively distant transmitting stations. This invention incorporates an improved signal-collecting means.

' Signal-collecting systems generally include a resonant circuit tunable over a desired range of frequencies and classifiable as series or parallel resonant circuits depending upon how the signal voltage is generatedin or applied to the circuit. Collector systems of the-series type usually employ a so-called loop antenna or its equivalent to intercept the signals. It is to thistype that the present invention is addressed. 1 I

In their practicable forms; series collector systems have heretofore'been tuned by variation of the circuit capacitance. In the present application, and in my co-pending applications, Serial Numbers 319,671, 319,672. and 319,673, all'filed on. February 19, 1940; to which I shall refer in greater detail later in this specification, I disclose highly] advantageous and commercially practicable series collector systems tuned by variation of the circuit inductance, preferably by the employment of a ferromagnetic core of suitable characteristics movable relatively to an inductance coil in series with the loop or other exposed inductive element of the system. This method of'tuning has the additional advantage of providing means for controlling the highfrequency resistance of the system in substantially any desired manner as the system is tuned over the frequency range. Resonant circuits tuned by inductance variation by means of ferromagnetic cores movable relatively to the inductive element in the circuit, and possessing the advantage of simultaneous control of the circuit resistance are disclosed by Polydoroif in United States Patent No. Re. 21,282, in which a resonant circuit having an inductance coil and capacitor is adjusted over a range of frequencies by movement of a compressed comminuted core relatively to the inductance coil. This method of tuning is called permeability tuning. An improved form of such a system is disclosed 2,051,012. Both Polydoroffs original system and my improvedsystem readily cover an adequate range of frequencies and may easily be ganged to provide multiple unit systems. In general, the signal voltage generated in a upper end of the broadcast range will provide approximately three times as much signal voltage in the collector system as a signal at the inmy United States Patent No.

lower end of this range. 1 Various expedients' have been employed in broadcast receivers in an effort to compensate for this inherent deficiency of the collector system. 'In my co-pending. applications above referred to, and in the present application, I disclose collector systemsin' which this deficiency is avoided by employing simultaneous variation 'of circuit inductance and resistance, preferably asprovided by permeability tuning, inparticular circuit arrangements In accordance with application. Serial Number 319,672; filed February lil, 1940-; I employanjadditional exposed inductive element in an untuned portion ofthe system, In accordance with application Serial Number 319,673, filed February 19, 1940, Iemploy an additional unexposed inductive element arranged to regeneratively increase the delivered voltage in a desired manner. through a coupling variation also simultaneously produced by the relatively movable .ferromag netic core in a permeability-tuned circuit. In accordance with the present invention, I employ a transformer between the loop orother exposed element and the first vacuum tube in the radio receiver, and I arrange this transformer in such a waythat'm'ovement of' a ferromagnetic core relatively to its windings simultaneously tunes. the" resonant circuitwhichl supplies signal voltage t'o thefirst vacuum tube and a second'resonant' circuit which includes {the loop or other exposed; m An object of my invention is to providefan improved r vers-1 V I additional object is to provide a signalcollecting means which may be successfullyemq ployed in the most compact formsof radio re-' ceivers, and ,which may be placed in close proximity, to the receiver chassis without serious detriment. I

sun another object or the invention is to pro-.

vide a signal-collecting circuit whose perform-. ance characteristics may be readily controlled with respect to variation with frequency.

It is also an object of the invention-to provide the advantages of this realized.-

'It is a principal object of my present inven-, tion to provide a signal-collecting system in which the total gain is high and in which the delivered voltage is substantially independent of the fre quency of the intercepted signal. I

It; is a further object of my present invention, toprovide a signal-collectingcircuit towhich may be connected a loop or other exposed element of any reasonable dimensions and" inductance;

signal-collecting means for radio remanner which will be readily understood .from

the following description taken in connection with the drawings, in which:

Fig. 1 is a schematic diagram of a basic arrangement in accordance with the invention;

Fig. 2 is an equivalent circuit diagram in accordance with Fig. 1;

Figs. 3, 4 and 5 are modifications of the arrangement of Fig. 1;

Fig. 6 is an equivalent circuit diagram illustrative of the behavior of Fig. 1 under certain choices of the constants thereof;

Fig. 7 is a further modification of the arrangement of Fig. 1; and

Fig. 8 is an equivalent circuit diagram in accordance with Fig. 7.

In the arrangement according to Fig. 1, a loop or other suitable inductive element I is arranged to intercept the passing signals, and is directly connected to the primary 2 of a transformer whose secondary 3 forms a resonant circuit with capacitor 5, this circuit being connected in the conventional manner to supply signal voltage to input electrode or grid 1 of vacuum tube 8, which may be the first vacuum tube in a radio receiver, and which .may be arranged to act as a highfrequency amplifier, as a detector or as a modulator in a superheterodyne system. Ferromagnetic core 6 is movable relatively to primary 2 and secondary 3, and thus serves to simultaneously vary their inductances, their resistances and the mutual inductance of the transformer, indicated by numeral 4 and the bracket.

In a preferred embodiment according to Fig. 1, for which I shall later give appropriate circuit constants, primary 2 has a relatively small number of turns and is positioned over secondary 3 and at the grid end thereof, core 6 being arranged to enter the grounded end of the windings. In this manner a very considerable increase in the mutual inductance 4 occurs as the core 6 is advanced to tune the system to the lower frequencies. Stated in other words, transformer 2-3 is provided with a falling frequency-coupling characteristic which substantially compensates for the inherently rising frequency-voltage characteristic of the loop I.

As is well known, the effective high-frequency resistance of a permeability-tuned circuit increases as the core is advanced into the winding to tune the circuit to lower frequencies. Depending upon the materials employed and the process of producing the core, this increase in resistance may be controlled, and, if desired, may be made quite small. It is impractical, however, to attempt to entirely compensate for the lower voltages generated by the signal at the lower fre quencies, by employing a very low-loss core material and construction. 'As stated by Jacob in United States Patent No. 2,153,622, a series resonant circuit may be arranged to produce constant gain by employing a ferromagnetic core which will maintain the ratio of inductive reactance to resistance in the circuit, that is the circuit Q, substantially constant. In carrying out my invention I prefer to employ such a core, and to so arrange the coils 2 and 3 that the variation in coupling produced by the movable core 6 will substantially compensate for the lower voltages generated by the lower frequency signals. Such an arrangement is provided by the circuit constants for a practical embodiment which I shall later give. It will be understood, however, that since the system provides two performance controls, namely, the resistance variation and the coupling variation, both provided automatically by movement of the core, other elections may be made within the scope of the invention. For example, if a core is to be employed which, with the chosen coil, produces some performance variation other than that corresponding to constant circuit Q, then coil 2 may be so chosen with respect to its winding and position relatively to coil 3 and moving core 6 as to produce a compensatory variation of the coupling between coils 2 and. .3. Additionally, and also by virtue of the two controls provided by the invention, any reasonable desired relation of generated and delivered signal voltages may be secured.

In a successful embodiment of the invention according to Fig. 1, exposed element I comprised a -turn loop of No. 28 plain enamelled wire on a frame 5.25 inches by 11.25 inches by one inch and had an inductance of 1738 microhenries. Primary 2 comprised a universal wound cell of 80 turns of 7-44 single silk enamelled Litz wire with a winding length of inch and wound directly over secondary 3, which was a progressive univeral winding 1?; inches long and comprised 440 turns of 1544 single silk enamelled Litz wire with an internal diameter of 0.220 inch. Primary 2 had an air-core inductance of microhenries and a Q of 100 at 1500 kilocycles. Secondary 3 had an inductance value of 222 microhenries and a Q of at 1500 kilocycles. I

Referring to Fig. 2, which is an equivalent circuit diagram corresponding to Fig. 1, it may be shown that the resonant frequency of the system is in which the L, and C terms are the correspondingly numbered inductances and capacitances, respectively, in Fig. 1 and that the Q of the system is X3 AZ(XI+X2) Q- A2136 in which the X terms are the reactances of the correspondingly numbered inductances in Fig. 1 as indicated in Fig. 2, A=L4/(L1+L2) and Re and R1 are the effective series resistances, respectively, of the primary and secondary portions of the system. The effective input voltage will be and when R6 is small compared to X1 and X2, as is normally the case E'=E1A (4) If the inductance of loop is made considerably greater than the inductance of primary 2, and

proper choice of the material and construction of core 6, resistances Re and R7 may be made to have such variation as to maintain the Q of the system substantially constant, or alternatively for special purposes, any other reasonable desired frequency-Q characteristic may be secured. In the successful embodiment above described, core 6 was constructed of low-loss ferromagnetic powder insulated and compressed with a suitable binder in the well-known manner, and had a diameter of 0.2 inch and a length of 1% inches. In the same embodiment, the mutual inductance was varied by the core from 85 microhenries at 1500 kilocycles to 185 microhenries at 600 kilocycles.

Referring to Fig. 3, by employing a small coupling capacitance l2 connected between the high-potential sides of primary l and secondary II, an increase in the resonant gain of the system, and an improvement in the uniformity of performance over the frequency range, may be achieved. .The circuit of Fig. 3 is otherwise simi lar to that of Fig. 1. In the illustrative embodiment for which circuit constants are above given, capacitance l2 might appropriately have a value of micromicrofarads.

Fig. 4 shows another modification of the system in which the loop I5 is grounded at its electrical center 20 rather than at its low-potential terminal as in Fig. 1. As is well known, this balances the loop with respect to its capacitive field and thus to a very considerable extent prevents the loop from developing any signal voltage from the electrostatic component of the signal wave. Such an arrangement may be advantageous in cases where the capacitive pick-up of the loop proves to be the principal source of disturbing noises. Alternatively, the mid-point of the primary of the transformer may be grounded, as shown at 26 in Fig. 5, with the same effect.

In the previous discussion of Fig. 1, and its equivalent circuit, Fig. 2, it was assumed that the mutual inductance L4 did not exceed either the primary inductance L2 or the secondary inductance L3, so that the equivalent circuit reactance was always positive. However, by materially increasing the coupling, for example by increasing L3 and by placing winding 2 closer to winding 3 and distributed over it, the reactance reflected from the secondary circuit into the primary circuit will appear as a negative reactance or capacitance whose value varies with the position of core 6. Under these circum stances, and with proper design, the primary circuit containing the loop may be tuned in synchronism with the secondary circuit. This condition is illustrated in the equivalent circuit diagram of Fig. 6 in which the reactance reflected into the primary circuit is Xz-X4. It will be understood that the constants for the modified arrangements of Figs. 3, 4 and 5 may be so chosen as to secure the result just described in either of these modifications.

A somewhat more convenient arrangement for securing negative reactance tuning of the primary circuit including the loop is shown in the modification of Fig. '7, the equivalent circuit diagram of which is given in Fig. 8. In this arrangement, one terminal of loop 2'! is connected to the high-potential terminal of secondary 29, and the other terminal of loop 21 is connected to the high-potential terminal of primary 28. Referring to Fig. 8, the negative reactance Xzs-Xsa is in series with loop reactance X21, and these two reactances are shunted by the mutual reactance X32. Comparison of Figs. 6 and 8 shows that the circuits of Figs. 1 and 7 are closely alike, when the coupling is high, but experiment has shown that the arrangement of Fig. 7 is to be preferred when negative-reactance tuning of the loop circuit is to be secured.

The improved signal-collecting system according to the present application includes a closed circuit having an inductive portion, which may be a loop, exposed to the signals; and an unexposed inductive portion coupled to the inductive element of a second closed circuit having a fixed or'adjustable' capacitance, and arranged to supply signal voltage to the first vacuum tube of a radio receiver. The system is tuned to resonance with any desired signal within a wide range of frequencies solely by variation of the inductance values of the unexposed inductive portions, this variation being produced, for example, by means of a ferromagnetic core movable relatively to the unexposed inductive windings. If desired, I may secure synchronous tuning of both circuits by'employing sufiicient coupling to cause a negative reactance to be re-' netic core relatively to the unexposed windings.-

Having thusdescribed my invention, what I claim is:

1. A signal-collecting system for radio receivers and the like tunable to signals within a range offrequencies, including an element exposed to said signals and inductively reactive throughout said range, a non-variable capacitance, means for selecting a desired signal comprising a secondary winding connected across said capacitance to form a resonant secondary circuit and a ferromagnetic core movable relatively to said secondary winding to vary the inductance thereof, and means for inducing in said secondary winding a voltage proportional to the strength but substantially independent of the frequency of a selected signal comprising a primary winding connected directly across said element to form a-closed pri-' mary circuit and so positioned relatively to said secondary winding that for all positions of said core a capacitive reactance adequate to tune said primary circuit approximately to the frequency of said signal is reflected from said secondary circuit into said primary circuit.

2. A signal-collecting system for radio receivers and the like tunable to'signals within a range of frequencies, including an element exposed to said signals and inductively reactive throughout said range, a non-variable capacitance, means for selecting a desired signal comprising a secondary winding connected across said capacitance to form a resonant secondary circuit and a ferro-' magnetic core movable relatively to said secondary winding to vary the inductance thereof, said corehaving such loss characteristics as to maintain the resonant gain in said secondary circuit substantially constant, and means for inducing in said secondary winding a voltageproportional to the strength but substantially independent of the frequency of a selected signal comprising a primary winding connected directly across said element to form a closed primary circuit and so positioned relatively to said secondary winding that for all positions of said core a capacitive reactance adequate to tune said primary circuit approximately to the frequency of said signal is reflected from said secondary circuit into said primary circuit.

3. A signal-collecting system for radio receivers and the like tunable to signals within a range of frequencies,including-an element exposed to said signals and inductively reactive throughout said range, a non-variable capacitance, means for selecting a desired signal comprising a secondary winding connected across said capacitance to form a resonant secondary circuit and a ferromagnetic core movable relatively to said secondary winding to vary the inductance thereof, and means for inducing in said secondary winding a voltage proportional tothe strength but substantially independent of the frequency of a selected signal comprising a primary winding connected with said element to form a primary circuit and so positioned relatively to said secondary winding that for all positions of said core a capacitive reactance adequate to tune said primary circuit approximately to the frequency of said signal is reflected from said secondary circuit into said primary circuit.

4. A signal-collecting system for radio receivers and the like tunable to signals within a range of frequencies, including an element exposed to said signals and inductively reactive throughout said range, a non-variable capacitance, means for selecting a desired signal comprising a secondary Winding connected across said capacitance to form a resonant secondary circuit and a ferromagnetic core movable relatively to said secondary winding to vary the inductance thereof, and means for inducing in said secondary winding a voltage proportional to the strength but substantially independent of the frequency of a selected signal comprising a primary winding having a materially lower self-inductance than, and connected with said element to form a primary circuit and so positioned relatively to said secondary winding that for all positions of said core a capacitive reactance adequate to tune said primary circuit approximately to the frequency of said signal is reflected from said secondary circuit into said primary circuit.

5. A signal-collecting system for radio receivers and the like tunable to signals Within a range of frequencies, including an element exposed to said signals and inductively reactive throughout said range, a non-variable capacitance, means for selecting a desired signal comprising a secondary winding connected across said capacitance to form a resonant secondary circuit and a ferromagnetic core movable relatively to said secondary winding to vary the inductance thereof, and means for inducing in said secondary winding a voltage proportional to the strength but substantially independent of the frequency of a selected signal comprising a primary winding having a materially lower self-inductance than, and connected directly across said element to form a closed primary circuit and so positioned relatively to said secondary Winding that for all p0- sitions of said core a capacitive reactance adequate to tune said primary circuit approximately to the frequency of said signal is reflected from said secondary circuit into said primary circuit.

6. A signal-collecting system for radio receivers and the like tunable to signals within a range of frequencies, including an element exposed to said signals and inductively reactive throughout said range, a non-variable capacitance, means for selecting a desired signal comprising a secondary winding connected across said capacitance to form a resonant secondary circuit and a ferromagnetic core movable relatively to said secondary winding to vary the inductance thereof, said core having such loss characteristics as to maintain the resonant gain in said secondary circuit substantially constant, and means for inducing in said secondary winding a voltage proportional to the strength but substantially independent of the frequency of a selected signal comprising a primary winding connected with said element to form a primary circuit and so positioned relatively to said secondary winding that for all positions of said core a capacitive reactance adequate to tune said primary circuit approximately to the frequency of said signal is reflected from said secondary circuit into said primary circuit.

7. A signal-collecting system for radio receivers and the like tunable to signals within a range of frequencies, includingan element exposed to said signals and inductively reactive throughout said range, a non-variable capacitance, means for selecting a desired signal comprising a secondary winding connected across said capaci tance to form a resonant secondary circuit and a ferromagnetic core movable relatively to said secondary winding to vary the inductance thereof, said core having such loss characteristics as to maintain the resonant gain in said secondary circuit substantially constant, and means for inducing in said secondary winding a voltage proportional to the strength but substantially independent of the frequency of a selected signal comprising a primary winding having a smaller inductance value than, and connected with said element to form a primary circuit and so positioned relatively to said secondary winding that for all positions of said core a capacitive reactance adequate to tune said, primary circuit approximately to the frequency of said signal is reflected from said secondary circuit into said primary circuit.

8. A signal-collecting system for radio receivers and the like tunableto signals within a range of frequencies, including an element exposed to said signals and inductively reactive throughout said range, a non-variable capacitance, means for selecting a desired signal comprising a secondary winding connected across said capacitance to form a resonant secondary circuit and a ferromagnetic core movable relatively to said secondary winding to vary the inductance thereof, said core having such loss characteristics as to maintain the resonant gain in said secondary circuit substantially constant, and means for inducing in said secondary winding a voltage proportional to the strength but substantially independentof the frequency of a selected signal comprising a primary winding having a smaller inductance value than, and connected directly across said element to form a closed primary circuit and so positioned relatively to said secondary winding that for all positions of said core a capacitive reactance adequate to tune said primary circuit approximately to the frequency of said signal is reflected from said secondary circuit into said primary circuit.

WILLIAM A. SCI-IAPER. 

